This invention relates to a device for performing a photocoagulation operation on a biological tissue, especially on the fundus of an eye.
Devices of this type are known. Compare the Report of the 74th Convention of the German Ophthalmological Society (Bericht uber die 74. Zusammenkunft der Deutschen Ophthalmologischen Gesellschaft), pages 420-427, J. F. Bergmann-Verlag, Munich 1977. According to this publication, the known devices comprise a source of radiation, for example a laser, the radiation from which is concentrated on the area of tissue to be treated, and there brings about an alteration of the tissue, in particular a coagulation process. A color-change occurs during the coagulation process, in particular a white coloration, the degree of the color-change being a measure of the therapeutic effect. In order to measure the white coloration, light emitted by a second radiating device, operating under steady-conditions, is directed onto the coagulation site, and only that light of the second radiating device which is diffusely reflected at the said site is supplied to a device for measuring the radiant intensity of the reflected light. An oscilloscope, connected in series, records the change of the white coloration.
In carrying out a coagulation operation, account must be taken of the following facts. The coagulation parameters must be preset for each coagulation operation. These parameters are essentially the exposure time, the radiated power, the wavelength of the radiation, and the size of the area to be irradiated. The prior art devices include appropriate controls for adjustably setting each of these parameters before each coagulation process. However, the tissue characteristics can vary from one area to be irradiated to another such area even within the same eye. Consequently, these parameters should be adjusted in accordance with these variations in order to achieve similar effects.
Taking account of this fact, the state of the art is subject to several disadvantages which are both fundamental and practical in nature:
1. Only the coagulation operation which has already taken place is assessed, and no provision is made for intervening in the process while it is under way, or for prematurely switching off the exposure in the event of unsuitable radiation parameters. Excessively severe or excessively slight lesions can occur in the absence of a correcting intervention of this type.
2. The markedly variable tissue characteristics, which cannot be measured in vivo, make it impossible to define the "optimum coagulation parameters" prior to the coagulation operation, but if similar effects are to be obtained, account must be taken of the different tissue characteristics when selecting the coagulation parameters.
3. The variable tissue characteristics require special coagulation parameters for each individual coagulation operation, even, for example, within one eye, it being necessary to determine these parameters in advance, for each individual case, by experimental means. This experimental determination of the coagulation parameters is virtually impossible in the case of the generally large number of individual exposures in the course of a treatment (typically more than one hundred). This fact leads to the possibility that the coagulation operations may be performed too intensively.